Chimps, to the best of my knowledge, have not been seen to engage in
beastiality. I suspect it has been for them, as it is for us, something
that does not usually come to mind when one is in a sexual mood.
Present day human societys have A LOT of cultural *dreamwork* associated
sex; who you can marry and who you can't, arranged by who,various
rites to be preformed,bride price, virginity ect. Most of these shared
ideas
and rites associated with sex and pair bonding seem to exist to cement
family and group loyaltys. Beastiality is taboo because it confuses
these loyaltys. I doubt if homo erectus would have said exactly that but
they probably weren't very tolerent of beastiality if existing human and
chimp societys are anything to go on.